More simultaneous equations

A chemical manufacturer has an order for 500 litres of a 25% acid solution (i.e. 25% by volume is acid). Solutions of 30% and 18% are available in stock. The manufacturer wishes to make up the 500 litres from a mixture of 30% and 18% solutions.

Let x denote the amount of 30% solution required.

Let y denote the amount of 18% solution required.

Use simultaneous equations in x and y to determine the amount of each solution required.

I'm unsure how to set up the simultabeous equations therefore I am struggling with this question aby help would be appreciated!!